FIG. 1 is a circuit diagram of a conventional flyback voltage converter 10, in which a bridge rectifier 12 rectifies an AC input voltage VAC to produce a DC input voltage VIN, a transformer 16 has a primary winding Lp, a secondary winding Ls and an auxiliary winding La, a power switch SW is serially connected to the primary winding Lp, and a controller 14 switches the power switch SW to convert the DC input voltage VIN into an output voltage VOUT. FIG. 2 is a waveform diagram of the current Is in the secondary winding Ls and the voltage Vds across the secondary winding Ls. FIG. 3 is a waveform diagram of the peak value Is_pk of the current Is and the current conduction time Toff during which the current Is in the secondary winding Ls is greater than zero. Referring to FIGS. 1 and 2, during the power switch SW is on, for example, from time t1 to time t2 in FIG. 2, a current Ip is generated in the primary winding Lp, the voltage Vds across the secondary winding Ls has a negative value, and there is no current in the secondary winding Ls. At time t2, the power switch SW is turned off so that the voltage Vds across the secondary winding Ls changes to be positive and in consequence, the current Is is induced in the secondary winding Ls. Then, as shown by waveform 20, the current Is decreases slowly until it reaches zero at time t3. Assuming that the power switch SW has a switching cycle Ts, the current Is in the secondary winding Ls will have the average valueIo=0.5×Is—pk×Toff/Ts,  [Eq-1]where Is_pk is the peak value of the current Is. From the equation Eq-1, it is obtained the peak valueIs—pk=2×Io×Ts/Toff.  [Eq-2]Ideally, the output voltage VOUT of the flyback voltage converter 10 is constant, which requires the average current Io be constant and the switching cycle Ts of the power switch SW also be constant. Therefore, it can be known from the equation Eq-2 that the product of the peak value Is_pk and current conduction time Toff of the current Is in the secondary winding Ls is constant, or the peak value Is_pk is in inverse proportion to the current conduction time Toff. As shown in FIG. 3, the peak value Is_pk decreases as the current conduction time Toff increases.
Therefore, it is desired an approach for rendering the peak value Is_pk being in inverse proportion to the current conduction time Toff.